All vertebrate animals have a central axis of the body that consists of the spinal or vertebral column. The vertebral column consists of a number of connected irregular bones, termed the vertebrae, which surround and thereby protect a spinal cord. The vertebrae also support the weight of the trunk and transmit the weight to the lower limbs.
The vertebrae are grouped according to the region in which they lie—cervical, thoracic, lumbar, sacral and coccygeal or caudal. Each vertebra has a ventral and dorsal side. In series with each vertebra are a number of spinal nerves. Each nerve is formed by the union of an anterior (motor) and posterior (sensory) nerve-root. The posterior or dorsal nerve-roots are the central branches of the axons of the unipolar cells of the spinal ganglia. There are thirty-one pairs of spinal nerves in the human: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
Severe or disabling chronic pain is often observed after spinal cord injury (SCI). It is believed that these forms of pain have a central origin.
Current models of SCI are problematic in that they typically induce urinary retention, paresis/paralysis, and autotomy. Urinary retention and associated infections can be detrimental to the health of the animals, while motor dysfunction and autotomy make it very difficult if not impossible to cleanly assess for for exaggerated pain. Further, in deafferentation models to date, such as extradural posterior cervical rhizotomy (e.g., Lombard et al., Pain, 6:163-174, 1979), the only available indicator of pain is autotomy, owing to deafferentation of the territory of interest. Additionally, the hyperreflexia and spasticity observed in many models can further complicate the interpretation of experimental results.
There exists a need for valid animal models of central neuropathic pain, including below-level pain which is the most common and intractable neuropathic pain associated with SCI (Yezierski, Neurosignals, 14:182-193, 2005).